1. Field of the Invention
The present invention relates to an optical scanner used for reading bar codes and other symbols.
2. Description of the Related Art
Symbols such as bar codes and characters are conventionally read by optical means where a laser beam scans the surfaces on which the symbols are formed, causing the reflected light from the scanned surfaces to be received by an appropriate light-receiving device. The light-receiving device outputs signals that are associated with the intensity of the reflected light. Accordingly, when a bar code, which consists of black bars and white spaces, is scanned, small signals are produced for the bars and large signals for the spaces. Then, if the outputs of the light-receiving device, after optional amplification, are binarized through differentiation by an appropriate slice level, a binary signal that is associated with the particular bar code can be produced.
The light source for generating laser beams has heretofore been a He-Ne laser, but the use of semiconductor lasers has increased in order to reduce the size and weight of the overall equipment. However, since the laser light emitted from semiconductor lasers is highly diffusive, the output light is usually focused by a lens or other suitable optical members to create a substantially collimated laser beam. Bars in bar codes can be as narrow as 0.2 mm or less, and in order to resolve such fine-line bar codes with laser beams, the latter must be focused to a beam spot diameter of no more than 0.2 mm. Hence, the laser beam that is used on bar code and other symbol readers is not completely parallel light, but converging light having a focus point.
At its focus and in nearby areas, the converging laser beam has a small enough spot diameter to enable the reading of symbols with high resolution. On the other hand, symbol resolution deteriorates for symbols located at some distance from the focus of the laser beam. This inevitably causes the problem that a high resolution scan cannot be assured over a wide reading range, and in practice, bar codes that are formed on the surfaces of articles of different sizes may not be effectively read by a bar code or other symbol reader that is fixed above a conveyor belt transporting those articles. Stated more specifically, if the distance between the reader and the article's surface varies from one article to another, there is a high likelihood that the laser beam cannot be focused to an adequately small spot diameter on the article's surface, thereby reducing the scanner's effectiveness.
Two approaches have been taken in the art to solve this problem. The first solution is described in the documents of Japanese Patent Unexamined Publication Nos. Hei 2-170290, Hei 2-7182, etc. FIGS. 12(a) and 12(b) show part of the system configuration that is taught in Japanese Patent Unexamined Publication No. Hei 2-7182, supra. Light from a semiconductor laser 1 is focused by a condenser lens 2 to form a laser beam 3. The laser beam 3 has a beam waist BW at the focus with the focal length FL being determined by the relative positions of the light source 1 and the condenser lens 2. If a symbol such as a bar code is read in the position of beam waist BW, a maximum resolution can be attained. In the prior art system shown in FIGS. 12(a) and 12(b), the condenser lens 2 is adapted to be movable in the direction 4 which is parallel to the optical axis of the lens, so that the focal length FL, or the distance from the light source 1 to the focal position, can be reduced as shown in FIG. 12(a) or increased as shown in FIG. 12(b).
In the system configuration described above, the condenser lens 2 is moved in accordance with the reading distance (or the distance from the reader to the bar code carrying surface of an article) in such a way that a beam waist BW is formed either on the bar code carrying surface or in its neighborhood. This enables the bar code to be read at a high resolution irrespective of the reading distance. As a result, bar codes can be read effectively over a broad reading range. However, the response time to bring the laser light into focus is fairly long (say, 0.1 second). This can lead to failure in reading bar codes that are printed on the top surfaces of articles being transported on a conveyor belt. Stated more specifically, if two articles having different heights are transported with little or no space between them, there may be insufficient time to adjust the focus of the laser light from the position associated with the first article to the position associated with the next article. This can potentially cause failures in reading the bar code on the next article. A further problem occurs if two articles of different heights come to be located on a single scan line; in this case, the laser beam can be brought into focus on only one of the two articles and, hence, the bar code on the other article cannot be read at all.
In order to avoid these problems and to insure that the bar codes on consecutively transported articles are read positively, an adequate interval must be provided between adjacent articles. But then not only the efficiency of transporting the articles but also the process speed of reading bar codes will decrease.
Take, for example, the case where a response time of 0.1 second is required to change the focal position of a laser beam and where articles are transported at a speed of 120 m/min (=200 cm/sec), with the length of the reading region (the scan region extending in the direction of transport) being 40 cm; then, the minimum distance d, or the interval that is required between adjacent articles is d=200.times.0.1+40=60 cm. As one can readily understand, the efficiency of transporting the articles is very low.
The second approach of solving the problem of the prior art is described in Japanese Patent Unexamined Publication Nos. Hei 2-133891, 1-189782, etc. The systems taught in these references have the common feature of having a plurality of beam emitting means that have their foci set at different reading distances and which are selectively operated in accordance with the specific reading distance. This arrangement is also capable of changing the focal position with the reading distance and, hence, symbols can be read at high resolution over a wide reading range. The beam emitting means each contain a semiconductor laser as a light source and a condenser lens.
This low-efficiency problem is absent from the second approach of the solution since the focal position of laser light can be changed rapidly by selectively operating a plurality of beam emitting means in accordance with the specific reading distance. However, if bars in the bar codes to be read are 0.2 mm wide, the depth of reading that can be assured for one beam emitting means is only about 10 cm. Therefore, in order to read bar codes that are formed on the surfaces of articles of varying sizes, ranging from envelopes (with a height of about zero cm) to big containers or packages (with a height of about 100 cm), at least ten beam emitting means must be used. This increases the complexity of the optics in the symbol reader, making it costly and extremely difficult to design.
The present invention solves the above complications by providing an optical scanner that is simple in construction and, yet, is capable of efficiently scanning articles that are presented in the scan region at short time intervals.